1. Field of the Invention
The present invention relates to the art of growing single crystals and more particularly to producing single crystals of optical calcite.
The crystals produced in accordance with the process proposed herein, due to their high birefringence, find extensive application as a material for manufacturing polarizers of light, beam-diverging elements and, high-speed laser shutters.
2. Background Art
Known in the art is a process for producing single crystals of calcite under hydrothermal conditions in solutions of alkali metal chlorides and ammonia with the use of a glass insertion container at a temperature of 423-478K, a pressure of 15-25 MPa, temperature difference of 10-25K (N.Yu. Ikornikova et al., "Rost cristallow" (`Growth of Crystals`), 1961, vol. 4, Nauka Publishers, (Moscow), pp. 92-94), wherein the growth of crystals of calcite by a hydrothermal method is effected on seed plates arranged in the upper portion of an insertion container by virtue of supplying through the agency of thermal convection a saturated solution from the bottom portion of said container, wherein a batch material in the form of fragments of calcite crystals is disposed. Mass transfer occurs due to the creation of a temperature difference between the zones of the growth of crystals and the zone of disposition of crystalline calcium carbonate.
With the use of the prior-art process it is impossible to produce large single crystals of optical quality (with the thickness of the build-up of 12 mm and over) because of a reduction of the growth rate and formation of defects in the built-up layer in long-term cycles of crystallization.
Also known is a process of producing single crystals of calcite (Genet F. et al., High Temperatures-High Pressures. 1974, pp. 657-662), comprising recrystallization from solutions of NH.sub.2 Br at a growth temperature higher than 553K, at a pressure over 12 MPa, and at a temperature gradient over 5K.
In the course of heating an autoclave in a hydrothermal system due to hydrolysis of the solvent and dissolution of CaCO.sub.3 a gaseous phase is formed, consisting of CO.sub.2, NH.sub.3, H.sub.2. The presence of the gaseous phase leads to a spontaneous inventive nucleation of crystals, which hinders the deposition of the material on seed plates.
The growth of freely suspended seed plates is realized due to the formation of pyramids (build-up) sectors of all the faces that are possible under given physico-chemical conditions. Therefore, boundaries of the growth sectors are formed in the crystals, these boundaries being manifest as striae-like defects. These areas of the crystal, which occupy up to 50% of the volume of the built-up crystalline material, are rejected in the manufacture of optical elements. Monoblocks of crystals are considered to be good, if they contain no striae-like defects associated with the boundaries of the growth sectors. The growth of crystals is realized on freely suspended seed plates, i.e. the resulting crystals are composed of 6 pyramids (sectors) of the growth of faces of a cleavage rhombohedron (10-11). Therefore, boundaries of the growth sectors are formed in the crystals, these boundaries being optical defects which have to be rejected in the manufacture of optical elements. Only two pyramids of the growth of crystals, defined by the faces having a maximal surface area can be used effectively. For this reason, the prior-art process does not provide the obtaining of crystals with a maximal optically good monoarea.
Realization of the given process in long-term cycles of crystallization (over 100 days), required for producing large crystals suitable for industrial use, leads to slowing down and then to a complete inhibition of the growth of optical-quality crystals. This occurs due to the fact that in long-term cycles of crystallization there take place a considerable reduction of the surface of the dissolving solid phase and a considerable increase of the surface of growing crystals both because of expansion of the seed plates and because of an intensive growth of spontaneous-nucleation crystals. The formation of spontaneous-nucleation crystals is favoured by the origination of a gaseous phase in the course of the autoclave heating in the hydrothermal system due to hydrolysis of the solvent and dissolution of CaCO.sub.3.